1. Field of the Invention
The present invention relates to a method of generating color separation data and an image processing apparatus, and specifically, to generation of color separation data in the case that printing materials of particular colors are used for a printing apparatus.
2. Description of the Related Art
One of the major factors that affect a print quality in a printing apparatus is granularity of the printed image. It is known that in addition to printing materials which are generally used for printing, low concentration printing materials, whose concentration is lower than that of the generally used printing material, are used. For example, an ink-jet printing apparatus uses, for cyan and magenta inks, in addition to inks having dye or pigment concentration which is generally used, low concentration inks having lower dye or pigment concentration than that of generally used inks. Especially, a low density portion of an image such as a highlight portion of image is printed by using the low concentration inks to reduce granularity of the image.
On the other hand, when enlarging a gamut in a saturation or the like, it has been known that a printing material of particular color, which has a property of high saturation by nature, is used. When using the particular color printing material and making consideration of reducing the granularity of the image, generally, it is possible for the light printing material to be also used for the particular color. However, in this case, apparatus elements such as a printing head for the newly added printing material are additionally required, to cause a negative effect such as increasing the size of the apparatus. In contrast, Japanese Patent Application Laid-open No. 08-085219 (1996) reduces the granularity of an area printed with the particular color printing material without specifically using low concentration printing material of the particular color. More specifically, Japanese Patent Application Laid-open No. 08-085219(1996) uses light cyan and light magenta inks, which are low concentration inks respectively, instead of using particular blue ink, for low density portion in a gamut area which can be reproduced with the particular blue ink, and reduces the granularity in the low density portion.
However, though the particular printing material having high saturation and high lightness is used, a gamut area to be enlarged by nature with the particular printing material is not effectively used in Japanese Patent Application Laid-open No. 08-085219 (1996). More specifically, in the case of using the particular printing material, a gamut area can be enlarged in a L* axis direction, that is, in a lightness direction. However, in Japanese Patent Application Laid-open No. 08-085219 (1996), as shown in FIG. 12 of this document, a low saturation portion designated as hatched portion is reproduced without using the blue ink of the particular color printing material. For this reason, there may occur a problem that a gamut area to be enlarged for lightness of low saturation portion can not be effectively used.